An electrical multi-phase converter, i.e. a converter adapted for providing more than one output phase to a load, comprises phase branches, which may be individually switched to generate the respective output voltage for each phase. The phase branches may be assembled of converter cells that may have individual power supplies, for example individual DC links that are supplied by a rectifier per each converter cell.
For example, due to the power drawn from the DC links by the converter cells, which may vary due to the switching scheme of the converter cells, also the DC link voltage and in general the supply voltage of each converter cell may vary over time. In the case a reference voltage of a phase branch is very high and the available supply voltages are very low, the phase branch is not any more able to generate an output voltage as high as the reference voltage.
U.S. Pat. No. 5,625,545 shows an electrical drive, which provides three output phases. Each output phase is generated by series-connected converter cells.
In US 2013/148397 A1, a method is shown, in which reference output voltages are symmetrized within available potential zones of voltage sources. The symmetrizing common mode voltage is always added to the references even if none of the reference output voltages exceeds its potential zone.
“Modulation Strategies for Fault-Tolerant Operation of H-Bridge Multilevel Inverters”, P. Correa et al, 2006 IEEE International Symposium on Industrial Electronics deals with fault conditions in a converter with series connected converter cells.
US 2014/0078797 A1 relates to control of a cascaded multi-level converter in the case of a fault. A reference voltage is altered based on the number of active cells.
WO 20015/113780 A1 relates to switching of an inverter, which comprises series-connected converter cells. In the case of differing maximal amplitudes of the phase voltages, a virtual star point of the output voltages is shifted.